The present invention relates to an optical disc apparatus such as a compact disc (CD) apparatus and an optical file apparatus and the like, and more particularly to an optical disc apparatus that is improved with respect to control of an optical source. Controlling the beam light to be uniformly emitted from the optical source is important for the accurate reproduction of information in the optical disc apparatus.
FIG. 1 indicates a configuration of a conventional optical disc apparatus.
A chip 13 and a photodiode 18 for monitoring use are incorporated into a semiconductor laser element 11. The chip 13 emits a laser beam in the forward and rearward directions. A laser beam 14 that is emitted in the forward direction from the chip 13 is focused on a surface of an optical disc 12 after passing through a beam splitter 17 and an object lens 22. The laser beam 14 which is focused onto the surface of the optical disc 12 is reflected back in accordance with the information that is recorded on the optical disc 12. This reflected beam 16 is reflected by the beam splitter 17, passes through an optical system (not indicated in the figure) and is focused onto a light-receiving element (not indicated in the figure). The light receiving element outputs a signal in accordance with the reflected light 16 irradiated to it and the output signal from the light-receiving element is used as the basis for the reproduction of the information recorded on the optical disc 12.
A magnet 23 for use in writing information is provided facing the surface of the optical disc 12 to which the beam is not irradiated.
The laser beam 15 that is output in the rearwards direction from the chip 13 is irradiated to the photodiode 18. The photodiode 18 outputs a signal of a level in accordance with the intensity of the laser beam 15 which is irradiated to it. The output signal from the photo-diode 18 is supplied to an output control circuit 19. The output control circuit 19 outputs a control signal in accordance with the signal supplied from the photodiode 18. The control signal is then used as the basis for control so that the chip 13 emits a laser beam of uniform intensity.
A part of the reflected beam 16 from the optical disc passes through the beam splitter 17 without being reflected by it. This beam 20 (hereinafter termed the return beam) that has passed through the beam splitter 17 is irradiated to a semiconductor laser element 11, and a part of the return beam 20 is irradiated to a photodiode 18. The output of the photo diode is influenced by the return beam 20. Accordingly, the intensity of the laser beam from the chip 13 which is controlled on the basis of the signal output from the photodiode 18 is influenced by the return beam 20 and the intensity of the laser beam is therefore changed easily.
In general, an optical isolator is provided in front of the semiconductor laser element 11 so that it is possible for the return beam to be interrupted. However, for example, it is not generally possible to apply the optical isolator for optical disc apparatus for which it is necessary to irradiate the optical disc with a linearly polarized laser beam. In the optical disc apparatus for which such optical isolator cannot be applied, it is not possible to solve the problem of the intensity of the optical beam changing as has been described.